(a) Technical Field
The present invention relates to a hybrid fuel cell vehicle with a multi-power source and a multi-drive system and a method of controlling the same. More particularly, the present invention relates to a hybrid fuel cell vehicle having a plurality of power sources and a plurality of drive systems, by which overall reliability of the vehicle is ensured even when any one of the power sources or drive systems fails.
(b) Background Art
A fuel cell system is a power generation system that directly converts chemical energy of fuel into electrical energy, in which a pair of electrodes including an anode and a cathode is disposed on both sides of an electrolyte membrane such that electricity and heat are produced by an electrochemical reaction of ionized gas.
FIG. 14 is a schematic diagram showing a powertrain employed in a conventional fuel cell hybrid vehicle with a supercapacitor.
As shown in the figure, the fuel cell hybrid vehicle includes a fuel cell stack 100, a supercapacitor 120, an inverter 130, a motor 140, a reduction gear unit (RGU) 150, and a gear differential unit (GDU) 160.
The most widely used fuel cell stack 100 in a vehicle is a proton exchange membrane fuel cell (PEMFC) having a high power density. The process for generating electricity in the PEMFC is as follows.
Hydrogen gas at an anode of the PEMFC is dissociated into hydrogen ions and electrons by a reaction with a catalyst on the surface of a fuel electrode. The hydrogen ions pass through the electrolyte membrane to move to an air electrode disposed on the opposite side of the fuel electrode and, at the same time, the electrons produced by the reaction with the catalyst move along an external circuit, thus generating electricity.
The fuel cell stack 100 is a primary power source for driving the vehicle and forms a fuel cell system in which two 100 kW fuel cells are connected in series to each other. The supercapacitor 120 serves as an auxiliary power source capable of rapidly charging and discharging high power. The supercapacitor 120 supplements insufficient power of the fuel cell stack 100 with electrical energy stored therein and makes the maximum use of regenerative braking energy, thus allowing efficient use of the fuel cell.
The high voltage output from the primary power source and the auxiliary power source is converted from direct current to alternating current by the inverter 130 and supplied to the motor 140 (e.g., AC 240 kW) to drive the vehicle.
Here, since the two fuel cells 100 are connected in series to each other, a main bus terminal 102 is sustained at a high voltage (500 to 900 V), and the output torque of the motor 140 is firstly increased by the RGU 150 and further increased by the GDU 160 without use of any stepped transmission.
The speed reduction ratios of the RGU 150 and GDU 160 are configured to improve hill-climbing, acceleration and overtaking performance, which is well suited to a large-scale fuel cell vehicle including a bus.
In the fuel cell vehicle employing the fuel cells connected in series and the large-sized motor 140, the maximum speed of the vehicle is limited by the speed limit of the large-sized motor 140 and, in the event that the speed reduction ratio is decreased to increase the vehicle speed, the climbing performance may be deteriorated. On the contrary, in the event that the RGU 150 and the GDU 160 are designed to provide excellent climbing, acceleration, and overtaking performance of the fuel cell bus, the maximum speed of the motor 140 may be limited, and thus the vehicle cannot drive at a certain speed (e.g., 80 kph). Moreover, in the event that any one of the fuel cell modules goes to fail, entire fuel modules cannot be functions properly since the fuel cells 100 are connected in series to each other, and thus the supercapacitor 120 would be only available alternative to drive vehicle. However, since the supercapacitor 120 is configured to serves as a supplementary power, it cannot last for long time. Therefore, the vehicle should be taken to a repair shop to replace the abnormal fuel cell module. Accordingly, weakness of the fuel cell module with a serial structure is lack of reliability in an emergency.
FIG. 15 is a schematic diagram showing a powertrain of a fuel cell bus proposed by Toyota. The fuel cell bus comprises two set of powertrains each including a fuel cell system composed of a 90 kW fuel cell module 110, a DC-DC converter 124, a high-voltage battery 123, an inverter 131, and a motor 141. That is, the two fuel cell systems are electrically isolated, and the outputs of two motors 141 are mechanically coupled by a power coupling device (PCD) 154. Here, the PCD 154 has a structure in which a gear directly connected to rear wheels is engaged with a (spur) gear directly connected between the two motors 141 to transmit the output of the motors to the rear wheels.
Moreover, the fuel cell bus further employs two high-voltage batteries 123 as an auxiliary power source to increase power assist and regenerative braking energy of the high-capacity bus. Accordingly, with the use of the two electrically isolated fuel cell systems, two main bus terminals 112 are sustained at a low voltage (250 to 450 V), and the output torque of the motor 141 may be increased by a gear ratio between the RGU 150 and GDU 160 without use of any stepped transmission.
In case of a hybrid fuel cell bus employing a supercapacitor as an auxiliary power source, a component that has a large volume and is hard to control, such as the DC-DC converter used in the battery-fuel cell hybrid vehicle, is not required, but a precharge device for charging the supercapacitor during initial start-up is required. Accordingly, considering that the fuel cells occupy a significant portion of the manufacturing cost and layout of the fuel cell bus, there is a limit in increasing the maximum power of the motor due to the increase in manufacturing cost and the layout problem of the fuel cells.
Meanwhile, FIG. 16 shows a fuel cell bus employing a single fuel cell system 101 and a single large-sized motor 143, in which electrical power is supplied to the single large-sized motor 143 using only the fuel cell system 101 having a capacity of about 205 kW. Since the above fuel cell bus does not use any supplementary power source such as a battery or a supercapacitor, it is impossible to provide power assist, and thus the fuel cell system may be operated excessively, resulting in deterioration of durability of the fuel cell system. Moreover, since it is impossible to absorb regenerative braking energy, the energy efficiency may be reduced. Furthermore, since the single fuel cell 101 and the single large-sized motor 143 are used, it is difficult to ensure driving stability in an emergency where the fuel cell 101 or the motor 143 malfunctions.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.